Composite articles including prepegs, preforms, laminates and sandwich moldings, and methods of making the same

ABSTRACT

The present invention relates to high quality composite materials from fibers such as glass, polyaramid or graphite fibers, where the composite incorporates a polymer matrix embedding individual fibers. The composites are lightweight materials displaying enhanced strength and durability. In one aspect, the polymer matrix is a thermoplastic or other polymer type that cannot easily penetrate gaps between individual fibers by typical methods for thermosets. The invention also relates to methods for forming composite materials, where the fiber is exposed to an emulsion including polymer particles having sufficiently small dimensions to allow impregnation into the fiber gaps. Composite sheets and articles are also described, as well as the formation of new composites for porous articles, e.g., ceramics or wood, where a polymer matrix is embedded within the pores.

RELATED APPLICATIONS

[0001] This application is a divisional of U.S. application Ser. No.09/731,051, filed Dec. 6, 2000, entitled “Composite Articles IncludingPrepregs, Preforms, Laminates and Sandwich Moldings, and Methods ofMaking the Same,” by David A. Evans, which is a continuation ofinternational patent application serial no. PCT/US99/12621, filed Jun.8, 1999, entitled “Composite Articles Including Prepregs, Preforms,Laminates and Sandwich Moldings, and Methods of Making the Same,” byDavid A. Evans, which claims the benefit of U.S. provisional applicationserial No. 60/088,514, filed Jun. 8, 1998, entitled “ThermoplasticComposite Prepregs, Preforms and Moldings,” by David A. Evans. All ofthese applications are incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates to composite articles such as fiberresin composites having a polymer matrix embedding individual fibers,and composite prepregs having individual fibers with a coating ofpolymer particles. The invention further relates to methods for makingsuch composite articles, such as the use of an emulsion with polymerparticles of sufficiently small size to penetrate gaps betweenindividual fibers. The invention also relates to other compositearticles in which a porous material has a polymer matrix embedded withinpores of the material.

BACKGROUND OF THE INVENTION

[0003] There has been a long-felt need for lightweight fiber-resincomposite materials which display greater strength than currently knowncomposites. Fiber resin composites include fibrous articles, sheets andstrands (e.g., tow or yarn) which incorporate a polymer matrix embeddingfiber bundles or individual fibers of the article, sheet or strand. Onemajor application of these composites are materials for militaryairplanes.

[0004] There are two major types of fibers used in composites—choppedglass fibers and continuous fibers. Chopped glass fibers are used tomake composites of relatively lower strength. These composites containfrom 20% to 40% of fiber by volume, usually as a mat, as described inU.S. Pat. No. 3,713,962. Stiffer and/or stronger composites usecontinuous fibers in yarn form and contain more than 50% fiber byvolume. Examples of stiffer fibers include graphite, polyaramid orspecial glass fibers. As the volume of fiber in the composite increases,obtaining a uniform matrix between the fibers tends to be moredifficult.

[0005] Composites are often prepared via a “prepreg,” i.e. a compositeprecursor, in which the fibrous articles or strands are impregnated witha polymer matrix precursor mixture (e.g. U.S. Pat. No. 3,784,433).Prepregs are typically placed in a mold with the fibers positioned in adesired sequence and orientation and subsequently heated under pressureto fuse or polymerize the precursor components to form the polymermatrix of the final composite. The prepreg allows control of the resincontent and fiber orientation. Prepregs can be provided as collimatedtapes or fabrics.

[0006] One type of polymer matrix comprises thermoset plastics orresins. Thermoset resins typically are prepared from precursor mixturescomprising an oligomer and a crosslinking reagent. When heat or energyis applied, the precursor mixture reacts to form a hard,three-dimensional, cross-linked polymer matrix. Incorporating thermosetresins in composites is a relatively facile process because the startingcomponents of thermosets are either liquid resins or solutions of theprecursors. These are low viscosity liquids from 100 to 5,000 centipoise(0.1 to 5 Pa·s), which can rapidly wet the fibers. Yarns of glass,graphite or polyaramid are easily penetrated by the low viscosity resinto the core of the yarn, thus providing each fiber with a completecoating of polymer.

[0007] Thermoset composites suffer from several disadvantages. Lowmolding pressures are used to prepare these composites to avoid damageto the fibers. These low pressures, however, make it difficult tosuppress the formation of bubbles within the composite which can resultin voids or defects in the matrix coating. Thus, most processingproblems with thermoset composites are concerned with removing entrainedair or volatiles so that a void-free matrix is produced. Thermosetcomposites made by the prepreg method require lengthy cure times withalternating pressures to control the flow of the resin as it thickens toprevent bubbles in the matrix. Some high volume processes, such as resininfusion avoid the prepreg step but still require special equipment andmaterials along with constant monitoring of the process (e.g. U.S. Pat.Nos. 4,132,755, and 5,721,034). Thermoset polymers are not easy toprocess, regardless of whether the resin is applied to the yarns beforemolding or is infused into a preform of fibers. Although thermosetpolymers have enjoyed success as in lower performance composites, thedifficulties in processing these resins has restricted theirapplication.

[0008] To overcome some of the disadvantages of thermosets, the use ofthermoplastic resins as a polymer matrix in composites has beenattempted. Thermoplastic resins are long chain polymers of highmolecular weight. These polymers are highly viscous when melted and areoften non-Newtonian in their flow behavior. Thus, whereas thermosetshave viscosities in the range of 100 to 5,000 centipoise (0.1 to 5Pa·s), thermoplastics have melt viscosities ranging from 5,000 to20,000,000 centipoise (5 to 20,000 Pa·s), and more typically from 20,000to 100,000 centipoise (20 to 100 Pa·s). Despite a viscosity differenceof three orders of magnitude between thermosets and thermoplastics, someprocesses have been applied to both types of matrices for laminatingfibrous materials.

[0009] The combination of high viscosity (thermoplastics) and lowpressure (processes to avoid fiber breakage or distortion) is a majorsource of the molding problems with thermoplastic composites. Due to thehigh viscosity of thermoplastics, most of the processes to formthermoplastic prepregs involve coating the outside of the fiber bundleswith a thermoplastic polymer powder rather than coating individualfibers. The polymer powder is then melted to force the polymer around,into and onto the individual fibers. A few processes apply melt directlyto the fibers. A tape can be made by coating a dry tape of collimatedfibers with the polymer and applying a heated process that forces thepolymer into and around the fibers (e.g., see U.S. Pat. Nos. 4,549,920and 4,559,262). These processes involve a polymer of an exceptionallylow melt viscosity, such as polyetherketone (PEEK), as described in U.S.Pat. Nos. 4,883,552 and 4,792,481.

[0010] Other processes for incorporating thermoplastics in compositesinvolve preparing a thermoplastic slurry and melting and forcing theslurry onto the yarn (U.S. Pat. No. 5,019,427). A few thermoplastics canbe dissolved and introduced into the fiber bundle as a solution. Removalof solvent presents extra processing problems, however. Alternatively,U.S. Pat. No. 5,725,710 describes pretreating the fibers with a dilutedispersion to ease the passage of the melt polymer in a subsequentpultrusion step to make a tape prepreg. Another process involvescommingling, in which structural fibers such as graphite or glass aremixed with a thermoplastic fiber and the subsequent hybrid yarn is woveninto a fabric to be molded later (e.g., see U.S. Pat. Nos. 5,355,567,5,227,236 and 5,464,684). Separate yarns of thermoplastic andreinforcement containing many thousands of filaments, however, cannot bemixed mechanically in a one-by-one arrangement of each fiber. Thefibers, at best, are dispersed as smaller bundles. The laminatesproduced by this process typically contain areas that are resin rich andother areas that are mainly fiber and hence void-containing. Commingledthermoplastics are also restricted to only those polymers which formfibers.

[0011] In general thermoplastic composites have had limited success todate, due to a variety of factors including high temperatures, highpressures, and prolonged molding times needed to produce good qualitylaminates. Most of the efforts have been focused on combining highperformance polymers to structural fibers which has only exacerbated theprocess problems.

SUMMARY OF THE INVENTION

[0012] One aspect of the present invention is to provide an article, thearticle comprising a strand of a plurality of fibers and whereinsubstantially each fiber of the strand is coated by particles of apolymer.

[0013] Another aspect of the present invention provides an articlecomprising a strand of a plurality of fibers, where substantially eachfiber of the strand is embedded in a matrix derived from fused polymerparticles.

[0014] Another aspect of the present invention is a method for forming acomposite. The method comprises providing a strand comprising aplurality of fibers and exposing the strand to an emulsion includingpolymer particles. The method also comprises allowing the particles toform a coating around substantially each individual fiber.

[0015] Another aspect of the present invention is to provide a fibroussheet article comprising a plurality of strands, each strand comprisinga plurality of fibers, where substantially each fiber is embedded in amatrix derived from fused polymer particles.

[0016] Another aspect of the present invention provides a method forforming a composite fabric. The method comprises providing a fabriccomprising a plurality of strands where each strand is a plurality offibers. Substantially each of the individual fibers of the strands arecoated with polymer particles. The method also comprises fusing thepolymer particles to form a polymer matrix embedding substantially eachfiber.

[0017] Another aspect of the present invention provides an apparatus forforming a composite fabric. The apparatus comprises a first roll forsupplying a continuous first layer of strands, where each strand of thefirst layer is aligned along a first direction. The apparatus alsocomprises at least a second roll for supplying a continuous second layerof strands positioned adjacent the first layer thereby forming a fabric.Each strand of the second layer is aligned along a second direction,where the second direction is different from that of the firstdirection. The apparatus further comprises a reservoir containing anemulsion including polymer particles where the particles are capable ofcoating substantially each individual fiber of the strands of thefabric. A conveyor may also be provided to carry the fabric to and fromthe emulsion reservoir.

[0018] Another aspect of the present invention provides a method forforming a composite. The method involves providing an article havingpores and exposing the article to a polymer emulsion. The particles ofthe polymer are allowed to impregnate the pores of the article.

[0019] Another aspect of the present invention provides a compositecomprising a porous article having polymer particles impregnating thepores of the article.

[0020] Another aspect of the present invention provides a compositecomprising a porous article having a polymer matrix embedded within thepores of the article.

[0021] Other advantages and features of the invention will be apparentfrom the following detailed description of the invention when consideredin conjunction with the accompanying drawings, which are schematic andwhich are not intended to be drawn to scale. In the figures, eachidentical or nearly identical component that is illustrated in variousfigures is represented by a single numeral. For purposes of clarity, notevery component is labeled in every figure, nor is every component ofeach embodiment of the invention shown where illustration is notnecessary to allow those of ordinary skill in the art to understand theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a schematic cross-section of a composite article;

[0023]FIG. 2 is a schematic cross-section of a prior art prepreg;

[0024]FIG. 3 is a schematic cross-section of a prepreg having particlesof a sufficiently small size to impregnate gaps between individualfibers;

[0025]FIG. 4 is a theoretical, schematic fiber bundle in a cubicclose-packed arrangement, enabling a calculation of gap sizes; and

[0026]FIG. 5 is a plot of volume fraction of fibers having a cubicclose-packed arrangement (abscissa) versus gap size (ordinate).

DETAILED DESCRIPTION

[0027] The present invention relates to fiber composites having ahigh-quality polymer matrix, where the matrix is prepared by introducingpolymer as particles small enough to penetrate an area betweenindividual fibers. Subsequent fusion of the polymer particles results ina matrix embedding individual fibers, where the matrix is substantiallyfree of voids and defects. The present invention also relates to fabricshaving individual fibers coated with a polymer matrix, and to otherporous materials such as ceramics and wood that are capable of beingimpregnated by polymer particles which can be fused to form a matrix.

[0028]FIG. 1 shows a schematic cross section of a fiber composite 2.Composite 2 comprises individual fibers 4 embedded within polymer matrix6. To enhance composite strength, matrix 6 embeds individual fibers 4.Composite strength is generally dependent on the ability of the matrix 6to absorb much of the load that may be placed on articles comprisingthese fibers. Should matrix 6 include voids or other defects,schematically shown in FIG. 1 as matrix holes 8, such defects can leadto degradation of the structural integrity of the matrix 6, andultimately, loss of strength of composite 2.

[0029] An advantageous feature of the present invention involves theability to impregnate gaps between individual fibers of a fibrousarticle or strand with a polymer resin. Previous, strategies toimpregnate fibers with polymer resins, such as thermoplastics, involvedforming a melt and applying high pressure to force the polymer into thefiber cavities. High pressures are necessary, as polymer melts aretypically highly viscous fluids. Once the melt has impregnated thecavities, however, the melt can continue to flow, resulting in creationof unwanted voids or spaces in the resulting matrix. In certain casespolymers capable of dissolution in organic solvents can be introducedinto the fibers as a solution. However, subsequent removal of thesolvent can also lead to the formation of unwanted voids and defects.

[0030] As an alternative to forcing polymer melts through fibrousarticles, prepregs can incorporate polymer particles. But for manypolymers, particles obtained by common grinding methods do not have asufficiently small size to impregnate the gaps between individualfibers. Thus, these prepregs have a coating of particles outside fiberbundles, as opposed to impregnating individual fibers.

[0031]FIG. 2 shows a schematic cross section of a prior artthermoplastic prepreg 10 comprising thermoplastic particles 12, fibers14, and gaps 16 which exist between the fibers 14. Fibers such asgraphite, glass and ordered polymer fibers generally have an averagediameter ranging from 7 μm to 20 μm in diameter. Thermoplastic particles12 are usually prepared by dry grinding methods which lead to averageparticle sizes of 10 μm to 50 μm in diameter. Thus, the size of gap 16is typically smaller than the diameter of particles 12 in prior artthermoplastic prepregs. Due to these large particle sizes, thermoplasticparticles 12 are incapable of impregnating into gaps 16 between fibers14 in a facile manner.

[0032]FIG. 2 highlights the small gap sizes and difficulty inimpregnating these gaps with polymer melts. High pressures are needed toachieve such impregnation, as discussed previously. Such pressures canadd to already existing prepreg and composite processing problems. Inaddition, even if impregnation between individual fibers has beensuccessfully achieved with polymer melts, subsequent polymer flow cancreate detrimental voids.

[0033] Thus, one aspect of the present invention provides an articlecomprising a strand having a plurality of fibers, where substantiallyeach fiber in the strand is coated by particles of a polymer. Examplesof fibers include glass, graphite and ordered polymer fibers. Inreality, each and every fiber in the strand may not be coated byparticles of a polymer and thus “substantially each fiber in the strand”refers to coating at least 50% of the fibers, preferably at least 80% ofthe fibers are individually coated by particles, more preferably atleast 90% of the fibers are individually coated by particles, still morepreferably at least 95% of the fibers are individually coated byparticles, and even more preferably still at least about 99% of thefibers are individually coated by particles. The strand can be providedas a tow or as a yarn. Examples of ordered polymer fibers includeKevlar® and Twaron® polyaramide, polybenzimidazole, polybenzoxazole orpolybenzbisthiazole fibers. This aspect involves the formation of a“prepreg,” which refers to a precursor of a composite having a polymerresin precursor impregnated into the fibrous material. The resinprecursor can be a high molecular weight polymer and/or can include amonomer or oligomer. Subsequent treatment of the prepreg results information of the matrix. In one embodiment, the polymer is a solidpolymer.

[0034] In one embodiment, the polymer can exist as a high viscositypolymer having a viscosity of at least 5,000 centipoise (5 Pa·s),preferably at least 10,000 centipoise (10 Pa·s), more preferably atleast 50,000 centipoise (50 Pa·s), still more preferably at least100,000 centipoise (100 Pa·s) and even more preferably from 100,000 to20,000,000 centipoise (100 to 20,000 Pa·s). At these viscosity levels,the polymer is incapable of significant flow when incorporated as amatrix in the composite, i.e. the polymer is a non-flowing polymer. Thepolymer can be a homopolymer or a copolymer, such as a random copolymeror a block copolymer, and can exist as a syndiotactic or stereotacticform of these polymers as well as blends or alloys of any of thesepolymers.

[0035] In one embodiment, the polymer is a thermoplastic. Examples ofthermoplastics include polyolefins, polystyrene, polyamides (e.g.,nylons), polyketones, polyimides, polypropylene oxide,acrylonitrile-butadiene-styrene (ABS), polyacetals, polyesters,polyphenoxies, polyacrylic esters, polyvinyl esters, polyvinyl halides,polysiloxanes, polyurethanes, polyethers, polysulfides, polycarbonates,polybutylenes and polyarylates.

[0036] In one embodiment, the polymer is not a true thermoplastic, i.e.,further chemical reactions are required to form the final polymer. Thethermoplastic may contain a crosslinking agent in the polymer chain suchas a carboxyl group in an acrylic copolymer or it may be formulated withan external crosslinker such as a phenoxy with a multivalent amine.Whenever and however the polymer is crosslinked, the invention can stillbe applied to penetrate the strand with the corresponding polymerparticles and to subsequently fuse these particles into a matrix. If thepolymer is cross-linked, preferably the polymer is incapable of flow(i.e., non-flowing) when incorporated as a matrix in the composite.

[0037]FIG. 3 shows, as an embodiment of the present invention, aschematic cross-section of prepreg 20. Prepreg 20 comprises fibers 24and polymer particles 22, where polymer particles 22 have a sufficientlysmall diameter that allows particles 22 to impregnate the gaps 26between individual fibers 24.

[0038]FIG. 4 shows, as an embodiment of the invention, a schematicexpanded cross-sectional area of a fiber bundle. By assuming atheoretical cubic packing arrangement 30 of fibers in the bundle andknowing the average fiber diameter 31, a theoretical gap dimension 32can be calculated. In reality, the gap dimensions are variable, withsome dimensions being greater or smaller than the theoretical size.

[0039]FIG. 5 is a graphical representation of the gap between the fibersin a composite. In FIG. 5, the abscissa represents the volume fractionif occupied by fiber in the bundle having a cubic close-packedarrangement as in FIG. 4 and the ordinate represents the gap size inmicrometers (μm). In one embodiment, the bundle has a fiber volume of 40to 70% in the final composite. The theoretical minimum gap, g, betweenfibers is plotted in FIG. 5 for typical fiber sizes in a composite (seegap dimension 32 of FIG. 4). For example, for a fiber diameter of 20 μmand a volume fraction of 0.5, the theoretical gap would be approximately11 μm. Even for a volume fraction of as low as 0.4, a fiber having adiameter of 20 μm will have a theoretical gap of only 17 μm, less thanthe typical prior art particle dimensions of 20 to 50 μm and thuspreventing penetration of particles through these gaps.

[0040] The success of resin transfer molding and resin infusionprocesses for thermosets indicates that the flow of these low viscosityresins are helped by surface tension or “capillary” forces. When a lowviscosity resin is injected into a tightly packed preform of yarns,there is no pressure at the front of the resin as it advances throughthe preform. Preforms are completely infused by these thermosets despitethe lack of pressure. At the size of gap indicated in FIG. 5, capillaryforces would be significant to help introduce a liquid epoxy thermosetinto the gap. In fact, the smaller the gap, the higher the capillaryforce, and these capillary forces may be helping the impregnation ofthermosets into prepregs.

[0041] Because the viscosity of a melted thermoplastic is three ordersof magnitude higher and the surface tension of the melt will be very lowbecause of the high molecular weight, capillary forces are most likelyinsignificant for thermoplastics. Thus thermoplastic compositefabricators (of the prior art) have previously relied on externalpressure to force the resin into the fiber gaps. The gap between fibersis typically very small and would require high pressures to obtainsignificant flow of the polymer melt through the gaps. These highpressures could in fact force the fibers to close up on each other andfurther limit the penetration of the yarns by the resin. Externalpressures are typically higher for prior art thermoplastics—up to 500psi (3450 kPa) for molding. Only thermoplastics with lower viscositymelts (polypropylene, polyamides, polyesters and special grades ofpolyether ketone as in U.S. Pat. No. 4,549,920) have had limited successin composites.

[0042] Thus, in one embodiment the polymer particles used in the presentinvention have an average diameter of less than 0.25 times the fiberdiameter to allow facile impregnation of particles through the gaps. Inanother embodiment, the particles have an average diameter of less than5 μm, preferably less than about 1 μm, more preferably less than about0.5 μm, and more preferably still, less than about 0.25 μm.

[0043] Another aspect of the present invention provides an articlecomprising a strand comprising a plurality of fibers where substantiallyeach fiber is embedded in a matrix derived from fused polymer particles.Referring back to FIG. 3, prepreg 20 can be subjected to conditions ofheat and/or pressure that allow the particles 22 to be fused together toform a continuous polymer matrix to embed the individual fibers 24. Bythe use of prepreg 20 comprising individual fibers 24 coated withpolymer particles 22, a matrix can be achieved that is substantiallyfree of voids and/or other defects. A defect-free matrix cannot beaccomplished when a polymer melt is forced at high pressure in the gapsbetween the individual fibers due to continuing flow of the matrix oncethe melt has impregnated the fibers.

[0044] Another aspect of the present invention is a method for forming acomposite. The method involves providing a strand which comprises aplurality of fibers. The strand is then exposed to an emulsion includingpolymer particles. In this aspect, the emulsion includes polymerparticles of a size small enough to allow the particles to form acoating around substantially each individual fiber.

[0045] In one embodiment, the emulsion including polymer particles isprovided through emulsion polymerization. Emulsion polymerization iseffected when a water-insoluble monomer is placed in an aqueous solutionand allowed to polymerize, where polymerization is initiated by theproduction of free radicals in the water phase. Thus, the polymer can beany polymer capable of being formed by addition polymerization. In oneembodiment, the monomer is a liquid. In another embodiment, the monomerincludes a vinyl group. An example of such monomers include acrylic acidand esters, methacrylic acid and esters, styrene, acrylonitrile, vinylchloride, vinylidene, butadiene and others. Examples of polymers made byemulsion polymerization include polystyrene andacrylonitrile-butadiene-styrene (ABS) copolymers. Emulsionpolymerization results in polymer particles of small sizes, typicallybetween 0.1 μm to 0.25 μm, and these polymers have high molecularweights of at least 10,000 g/mol.

[0046] In another embodiment, the emulsion including polymer particlesis produced by a method of grinding polymer solids to a predeterminedparticle size, preferably by a wet grinding method. Preferably, thepredetermined particle size has dimensions described previously and isdictated by the fiber diameters. Polymers such as polyketones,polyesters, polyamides, polysulfones, polysulfides and others can beball mill ground to a stable emulsion by various methods known in theart.

[0047] Wet grinding methods such as ball-milling gives the opportunityfor introducing mineral or metallic materials for the modification ofthe properties of the final composite material. In one embodiment, thepolymer particles include an additive. Additives can be pigments forcoloring, flame retardants, inert fillers for cost reduction such asbentonite, metallic fillers such as silver for the control ofconductivity or fillers such as aluminum or silica powder for thecontrol of thermal expansion. Preferably the particles are as small asthe polymer particles. Providing the additives do not destabilize theemulsion, the composite can be varied by numerous parameters.

[0048] In one embodiment, an additive comprises a particle selected fromthe group consisting of ceramic and metallic particles. A small amountof polymer may carry a significant amount of ceramic or metallicparticles and carry those same particles into bundles of suitablefibers. An intermediate stage, containing the metallic or ceramic matrixpressed together with the polymer, gives a nearly net sized preform forthe final stage where the polymer is removed by burning off the polymerand the matrix is fused together. By this embodiment, it is possible toprepare ceramic or metallic composites. This embodiment overcomes manyof the problems of oxidation and contamination in handling very finepowders of ceramics or metals.

[0049] In another embodiment, the emulsion including polymer particlescan be prepared by precipitation from solution. The method of makingthese polymers is described in U.S. Pat. Nos. 3,993,843 and 4,222,918,both patents being hereby incorporated by reference in their entirety.

[0050] An advantageous feature of the present invention allows theindividual fibers to be coated with the polymer particles in the absenceof high pressures, thus avoiding certain prior art methods of forcingpolymer melts under pressure into the gaps between the fibers. In oneembodiment, each individual fiber is coated with particles by exposingthe strand to the emulsion in a manner that “wets” the strand with asufficient amount of the emulsion. The exposing can occur by immersingor dipping the strand in the emulsion, spraying the strand with theemulsion, painting the strand, or any other wetting means. Because theparticles in the emulsion have sufficiently small dimensions, simplyexposing the strand to the emulsion allows particles to impregnate thegaps between individual fibers.

[0051] In one embodiment, the method comprises fusing the particles inthe coating to form a polymer matrix around substantially eachindividual fiber. In one embodiment, the particles are fused by applyingan elevated temperature to the particles. The temperature is typicallyat least 125° C. In another embodiment, the particles are fused byapplying a pressure to the particles, typically pressures of at least 50psi (345 kPa). In one embodiment, both elevated temperatures and highpressures can be applied to fuse the particles.

[0052] Many polymers fuse as a continuous film if cured above atemperature known as the minimum film-forming temperature (MFT). The MFTis related to the glass transition temperature (T_(g)) of the polymer.Typically, MFT is less than the T_(g) and more preferably between 10° C.to 20° C. less than the T_(g). For example, a polymer has a T_(g) of atleast 80° C. and hence a high MFT of at least 60° C. Curing below theMFT results in a powder which can be easily lost from the prepreg. Inone embodiment, the polymer has a T_(g) of at least 50° C. and morepreferably at least 80° C.

[0053] The fusing step can be preceded by a drying step. Typically,drying is accomplished by air drying or by other methods known in theart. In another embodiment, the drying allows the polymer to fuse, e.g.,if the temperature of the latex is above the minimum MFT, thethermoplastic particles will fuse during the final stages of drying.

[0054] Fusing at lower temperatures may also be induced by adding smallamounts of a low T_(g) polymer by any method known in the art. In oneembodiment, the MFT can be lowered by using a coalescent or adding asecond polymer with a low MFT. Coalescents are a well-known art in latexcoating. A small amount of solvent is emulsified and added to the latexand has the effect of lowering the MFT during the critical film-formingstages. Examples of solvents suitable for styrenics and acrylics are1-phenoxy-2-propanol (Dowanol DPH) and Di-propylene butylene glycol(Dowanol DPNB). These are blended with water and 1% Aerosol OT at 50%until emulsified. Sufficient emulsion can be added to make the solventcontent between 1 and 3% in the latex.

[0055] By careful process control, the outside of a tape or fabricprepreg can be brought above the MFT while the core of the prepreg staysbelow MFT. The prepreg thus formed has a powder inside the yarns and afused layer on the outside of the yarn. This prepreg has flexibility andformability but does not lose resin during handling. Thus, in oneembodiment, an outer layer of the prepreg is fused where the fused outerlayer contains an inner core of unfused polymer particles Thisembodiment provides better fusing properties, for example when making athick heavy preform.

[0056] In one aspect the present invention affects how the final moldingis produced. The particles of polymer that are infused into the strandcan be simply fused together. There is no actual flow needed to make acomposite, as was necessary in the prior art. Thus it is possible that avery high molecular weight polymer can be used to make a composite wherethe polymer is substantially non-flowing, such as polymers havingmolecular weights of at least 5,000 g/mol and preferably at least 10,000g/mol. Such a situation exists with polytetrafluoroethylene where asolid polymer is made by sintering powder together at high temperatures.

[0057] Some emulsion polymers can incorporate cross-linking comonomersto improve the final polymer properties. Such copolymers can be used tomake composites by the invention even though the crosslinker may havereacted before the temperature is high enough for pressing.

[0058] Even linear thermoplastics can have high melt viscosities suchthat flow or spreading of the composite becomes difficult. Low viscositythermosets allow the yarn bundles to flatten during pressing when thoseyarn bundles are large—as in heavy weight fabrics. With prepregs of theinvention, the yarns will not flatten when pressed between parallelsheets. A pressure distributing medium is required to ensure thatpressure is maintained on all parts of the laminate. This distributingmedium can be a layer of rubber. A thickness of 0.75 to 1.5 mm ofsilicone rubber can be used for glass impregnates with weights up to 34oz/yd² (1.15 kg/m²). A layer of unreinforced polymer can also be usedand would have the added advantage of providing a finished side to thelaminate.

[0059] Another aspect of the present invention provides a fibrous sheetarticle. The article comprises a plurality of strands, each strandcomprising a plurality of fibers. Substantially each fiber is embeddedin a matrix derived from fused polymer particles, as describedpreviously.

[0060] In one embodiment, the prepreg is provided as a fabric, such as atape. In the prior art, because large particles were only attached tothe surface of the fiber bundles, the thickness of prepregs wasrestricted to what could be filled by the outside layer of resin. Anadvantageous feature of the method of the present invention is that theparticles can penetrate very thick layers of fabrics (e.g., up to 50oz/sq.yd (7 kg/m²) in weight) because of the capability of the polymerparticles to impregnate between individual fibers. For heavy fabrics, asingle ply of fabric can be formed, impregnated and molded to makeuseful moldings. Stitch-locked glass fabrics are in common use in theboating industry and these are economical. Glass fabrics with low-costlattices can provide moldings that compete in price with many othermaterials.

[0061] Heavy fabrics can also be impregnated on a continuous basis andformed into sheets. If the process is modified with a hot nip rollbefore the cold nip roll, the process can produce continuous sheetmaterials from heavy glass fabrics or other fiber forms. Rather thanrolling up the sheet, it can be cut to length with a flying saw. Thesheet material is useful as flat sheet stock. Because of thethermoplastic matrix, however, it can be reformed with heat into manymore useful products.

[0062] Another aspect of the present invention provides a method forforming a composite fabric. The method involves providing a fabriccomprising a plurality of strands, each strand comprising a plurality offibers. The strands are then positioned in a predetermined orientationby methods known in the art. Substantially each individual fiber of thestrands can be coated with polymer particles, preferably by exposing thearticle to an emulsion including polymer particles of sufficiently smallsize to penetrate gaps between individual fibers, as describedpreviously. The polymer particles can then be fused to form a polymermatrix around each fiber. The fusing may be accomplished under elevatedtemperatures and/or pressures. The fabric may be coated with anadditional layer, such as a nylon layer, to protect the fabric duringthe fusing step, if accomplished with an iron, and to prevent the fabricfrom sticking to the iron.

[0063] In one embodiment, prior to coating, the plurality of strands canbe formed into a fabric by a process selected from the group consistingof weaving, braiding, needle-punching, knitting and stitching. Inanother embodiment, these processes can be omitted as the finalcomposite has sufficient structural integrity without the extraprocessing steps.

[0064] In one embodiment, the fabric is provided in a continuousfashion. This providing step can comprise providing a first roll forsupplying a continuous first layer of strand. Typically, this layercomprises strands positioned side-by-side where each strand in the layeris aligned along the same direction. At least a second roll of strandcan be positioned adjacent the first layer of strands (e.g. on top ofthe first layer) where the second layer has strands positionedside-by-side in a different direction from that of the first layer ofstrands. An example of providing the fabric continuously via a pluralityof rolls is described in U.S. Pat. No. 5,055,242, hereby incorporated byreference in its entirety, where up to six rolls of different strandlayers can be provided to form the fabric. Each subsequent layerprovides strands aligned along different directions. For example, if afirst strand is provided in a direction of 0°, the next strand can beprovided in a direction anywhere between +45° and −45°. One of ordinaryskill in the art can select the desired direction of strands for eachlayer, depending on the particular application of the fabric. Uponformation of the fabric in a continuous fashion, the fabric can beexposed to the emulsion, the polymer particles can be fused to form amatrix and the fabric can be shaped by pressing the composite fabricinto a desired geometry.

[0065] Another aspect of the present invention provides an apparatus forforming a composite fabric. The apparatus comprises a first roll forsupplying a continuous first layer of strands and at least a second rollfor supplying a continuous second layer of strands positioned adjacentthe first layer (e.g. on top of the first layer) to form the fabric. Ofcourse more than two rolls can be used, the number of rolls beingdictated by the particular application of the fabric. As describedpreviously, each layer will have a different direction of strands froman adjacent layer. The apparatus can further comprise a reservoircontaining an emulsion including polymer particles, where the particlesare capable of coating substantially each individual fiber of thestrands of the fabric.

[0066] One advantageous embodiment of the apparatus provides a conveyorto carry the fabric to and from the emulsion reservoir. The conveyor canbe a conveyor belt or a series of pulley-like mechanisms or rollers thatallow a continuous sheet, arising from a roll, to be carried into theemulsion and out of the emulsion to be further processed. After theemulsion reservoir, the apparatus can include a press to shape thefabric. The apparatus preferably includes a heat source to fuse theparticles coating the substantially each individual fiber to form apolymer matrix embedding the individual fibers.

[0067] Another aspect of the present invention provides a method forforming a composite, starting with an article having pores. Variousporous articles can achieve increased structural properties by theaddition of a polymer matrix. The porous article can be a ceramic, woodor a foam. The article can be exposed to an emulsion including polymerparticles, such that the particles impregnate the pores of the article.Fusing the particles can result in a polymer matrix embedded withinpores of an article.

[0068] In one embodiment, two layers of the prepreg of the invention areconsolidated in a press and then arranged on each side of the porousarticle. If the porous article is treated with latex emulsion, thethermoplastic composite sheets can be bonded to it with heat and a lowpressure. The sandwich structure thus formed has many useful properties.

[0069] If the porous article is wood, composites of wood display betterstiffness and strength than the original wood. Foams can be treated withlatex to make lightweight sandwich structures. Foams and balsa wood areuseful cores for sporting goods, such as skis and snowboards, havingthermoplastic composite skins bonded to the core.

[0070] These and other embodiments of the present invention will be morefully understood from the examples below. The following examples areintended to illustrate the benefits of the present invention, but do notexemplify the full scope of the invention.

EXAMPLE 1

[0071] A styrene/acrylic copolymer emulsion (SA 204 from Para-Chem Inc.)was selected because of its properties. The polymer has a T_(g) (glasstransition temperature) of around 200° F. if prepared by non-emulsionmethods. If prepared by emulsion polymerization, SA204 has a T_(g) of50° F. The latex has particles around 200 nanometers in size.

[0072] Glass fabrics are coated generously by immersing the fabrics inthe emulsion and allowed to air dry for 16 hours. Once they areessentially dry, the sheets of prepreg are covered each side with anylon film. The film coated prepreg is then pressed with a smoothingiron set at 200° F. (93° C.) resulting in a fusing of the particles.(Small latex particles will fuse when taken above their glass transitiontemperature without melting).

[0073] Subsequently, the prepreg sheets are press molded into highquality laminates. Prepreg layers are stacked in the desired orientationand heat tacked together to maintain that orientation. The stack is thenloaded into a preheated press at 340-360° F. (171-182° C.). A pressureof 30 to 40 psi (207 to 276 kPa) is applied to the stack for 2 minutesand the press is then opened for 1 minute to allow any water (steam) toescape. Pressure is reapplied at 100 psi (689 kPa) on the stack for 2minutes. The press is then cooled as rapidly as possible to below 200°F. (93° C.) while maintaining pressure. Once the laminate is cooled, thepressure is relieved.

[0074] This process produces stiff, strong laminates with high fibercontent. The laminates can subsequently be reshaped by heating to 250°F. (121 ° C.) and forming to the desired shape. Providing 270° F. (132°C.) is not exceeded and the laminate is not bent too far, the integrityof the laminate is maintained during re-forming. Laminate properties aresufficient for many everyday applications. Woven and stitch-lockedfabrics of many styles have been successfully converted into laminatesby this technique. SA 204 prepregs are molded at temperatures andpressures which most composite molders can achieve.

[0075] The technique described above is a laboratory technique that canbe scaled up and automated. The process can be transferred to a textiletreater, preferably with a clip type tenter frame. The drying and fusingstages described above can be completed in one operation by passingthrough a drying oven at 200-300° F. (93-149° C.). Passing the fusedprepreg through a hot nip ensures that air is excluded and the fibersare pushed together. Thin fabrics will make prepregs that can be woundas rolls but heavy fabrics can be sheeted at the end of the treater.

EXAMPLE 2

[0076] Table 1 shows the strength properties of example fibrous articlesof the present invention. All properties are normalized to 60% fiber byvolume. The fabrics in Table 1 are all 18 oz (0.61 kg/m²) non-woven,stitched glass fabric with equal 0° and 90° fibers (BTI's 1800 style).The fabrics were coated with a thermoplastic latex as described inExample 1. The coating was dried at 250° F. (121 ° C.) for 30 minutes.Eight plies of the prepreg were then pressed together with each layerlaid up in the same direction. The plies were pressed together under 100psi (689 kPa) pressure at 360° F. (182° C.). TABLE 1 Fiber compositeproperties Manufacturer Apparent Flexural Apparent FlexuralThermoplastic type (Thermoplastic number) Strength (ksi) (MPa) Modulus(msi) (GPa) Vinyl acetate/Acrylic Air Products, Inc. 68.8 (474) 2.66(18.3) (Vancryl 989) Acrylic Parachem 71.2 (491) 3.60 (24.8) (Paracryl8444) Styrene/Acrylic Parachem 77.4 (533) 2.93 (20.2) (SA-204) AcrylicPolymer Latex Corp. 54.6 (376) 2.70 (18.6) (Rohamere 4010D)

EXAMPLE 3

[0077] Continuous graphite fiber tows (strands) are dipped into a bathof SA 204 latex while being spread over a plexiglass bar. The tows arethen passed through a tube oven with a wall temperature of 550-650° F.(288-343° C.). The tow spends about 30 seconds in the oven while thewater is flashed off and the latex particles are fused. As it exits theoven, the tow is passed through rollers to flatten it to a uniformthickness. Single tows can be treated (towpreg) or collimated groups oftows can form tapes. Tape and towpreg forms of the thermoplasticcomposite lend themselves to automated molding processes such as hotwinding and automatic fiber placement. Sheets of the tape can be stackedin controlled orientations and pressed into useful laminates with thesame press cycle as described above for glass prepregs.

EXAMPLE 4

[0078] SA 204 and three more latex types are applied to a 0/90 stitchedglass fabric of 18 oz (0.61 kg/M²) weight. The prepreg sheets are driedin an oven at 250-300° F. (121-149° C.) for 30 minutes. Eight sheets ofeach prepreg type are stacked and pressed to the press cycle asdescribed in Example 1.

EXAMPLE 5

[0079] Although the prepregs of the invention will be assembled intopreforms and pressed much faster than the competing epoxy systems,moldings with large compound curvatures may be slower to produce becauseeach ply may need some preshaping before placing in the mold. Thisexample eliminates the prepreg step and the forming of those prepregs.This makes the invention even more compatible with existing thermosetprocesses.

[0080] The shape of one side of the mold is approximately reproducedwith a permeable material such as woven wire or perforated metal. Theother side of the mold is also reproduced in woven wire or similarmaterial that is reasonably rigid but will allow liquids to flowthrough. Layers of dry fabric (in the sizes and orientations required bythe mechanical design) are then laid into the first shell of woven wire.The second shell of woven wire is then placed over the fabric layerssuch that the fabric layers are trapped between the woven wire layersand are formed to approximate the shape of the final molding. Thispreform of fabric layers is soaked and coated with a thermoplasticpolymer latex. This preform is dried. The entrained polymer is fused ifrough handling or prolonged storage is required. A preform is thusproduced which is almost the final shape and can drop straight into themold.

EXAMPLE 6

[0081] A stitched fabric with 18 oz/sq yard (0.61 kg/m²) of continuousglass strands arranged with equal amounts in the 0° (warp), +45° and−45° directions is coated on a continuous basis. The fabric is dippedinto an acrylic emulsion (Parachem 8444) with a solids content of 45%and a particle size of 200 nanometers. At a throughput of 55 yards/hour(0.14 m/s) the fabric picked up 38% solids by weight. At 110 yds/hour(0.28 m/s) the fabric picked up about 35% solids. The fabric was driedby passing through a 15 foot (4.5 m) oven at 350° F. (177° C.) and thena 10 foot (3 m) oven at 250° F. (121° C.). With this drying regime, askin was formed on the outside of the prepreg by fusion of the acrylicwhile the bulk of the acrylic inside dried to a powder. This form allowsmaximum deformability of the prepreg while retaining polymer duringhandling. For maximum control of polymer weight all the polymer can befused but then the prepreg becomes more difficult to handle.

EXAMPLE 7

[0082] The latex of Example 5 is applied to end-grain balsa and allowedto soak in. The acrylic coating is dried onto the balsa and then twoprepressed plies of glass/acrylic are stacked each side of the balsa. Areduced pressure of 40 psi (276 kPa) and a temperature of 350° F. (177°C.) produces a sandwich panel and provides excellent peel properties.

[0083] Those skilled in the art will appreciate that all parameterslisted herein are meant to be examples and that actual parameters willdepend upon the specific application for which the methods and apparatusof the present invention are used. It is, therefore, to be understoodthat the foregoing embodiments are presented by way of example only andthat, within the scope of the appended claims and equivalents thereto,the invention may be practiced otherwise than as specifically described.

1. An article comprising: a strand having a plurality of fibers,substantially each fiber of the strand being coated by particles of apolymer wherein the area between fibers is substantially filled by thepolymer particles.
 2. The article of claim 1, wherein the polymer is athermoplastic.
 3. The article of claim 2, wherein the thermoplastic isselected from the group consisting of polyolefins, polystyrene,polyamides, polyketones, polyimides, polypropylene oxide,acrylonitrile-butadiene-styrene, polyacetals, polyesters, polyphenoxies,polyacrylic esters, polyvinyl esters, polyvinyl halides, polysiloxanes,polyurethanes, polyethers, polysulfides, polycarbonates, polybutylenes,polyarylates and random copolymers, block copolymers, syndiotacticpolymers, stereotactic polymers thereof and blends and alloys thereof.4. The article of claim 1, wherein the particles have an averagediameter of less than 0.25 times the fiber diameter.
 5. The article ofclaim 1, wherein the particles have an average diameter of less than 5μm.
 6. The article of claim 1, wherein the plurality of fibers isselected from the group consisting of glass, graphite and orderedpolymer fibers.
 7. The article of claim 1, wherein the polymer has aT_(g) of at least 50° C.
 8. The article of claim 1, wherein the polymerhas a viscosity of at least 5 Pa·s.
 9. The article of claim 1, whereinthe polymer is a non-flowing polymer.
 10. An article comprising: astrand having a plurality of fibers, substantially each fiber of thestrand being embedded in a matrix of fused polymer particles, the matrixbeing substantially free of voids.
 11. A method for forming a composite,comprising: providing a strand comprising a plurality of fibers;exposing the strand to an emulsion including polymer particles; andallowing the particles to form a coating around substantially eachfiber.
 12. The method of claim 11, wherein the polymer particles areprepared by emulsion polymerization.
 13. The method of claim 11, whereinthe polymer particles are prepared by grinding a solid polymer to apredetermined particle size.
 14. The method of claim 11, wherein theemulsion including the polymer particles are prepared by precipitationof the particles from solution.
 15. The method of claim 11, furthercomprising fusing the particles in the coating to form a polymer matrixaround substantially each fiber.
 16. The method of claim 15, wherein thepolymer matrix is substantially free of voids.
 17. The method of claim15, wherein the fusing comprises applying an elevated temperature to theparticles.
 18. The method of claim 15, wherein the fusing occurs free ofpolymer flow.
 19. The method of claim 17, wherein the elevatedtemperature is at least 125° C.
 20. The method of claim 15, wherein thefusing comprises applying a pressure to the particles.
 21. The method ofclaim 20, wherein the pressure is at least 345 kPa.
 22. The method ofclaim 17, wherein the fusing further comprises applying a pressure. 23.The method of claim 11, wherein the particles have an average diameterof less than 0.25 times the fiber diameter.
 24. The method of claim 11,wherein the particles have an average diameter of less than 5 μm. 25.The method of claim 11, wherein the polymer particles include anadditive.
 26. The method of claim 25, wherein the additive is selectedfrom the group consisting of a dye, a flame retardant, a filler for thecontrol of thermal expansion, a filler for the control of conductivityand a filler to lower cost.
 27. The method of claim 25, wherein theadditive is selected from the group consisting of a ceramic particle anda metallic particle.
 28. The method of claim 27, further comprisingburning off the polymer particles to form a composite selected from thegroup consisting of a ceramic composite and a metallic composite.
 29. Afibrous sheet article, comprising: a plurality of strands, each strandhaving a plurality of fibers and substantially each fiber being embeddedin a matrix of fused polymer particles, the matrix being substantiallyfree of voids.
 30. A method for forming a composite fabric, comprising:providing a fabric comprising a plurality of strands, each strand havinga plurality of fibers; coating substantially each fiber of each strandwith polymer particles; and fusing the polymer particles to form apolymer matrix embedding substantially each fiber, the matrix beingsubstantially free of voids.
 31. The method of claim 30, furthercomprising positioning the strands in a predetermined orientation priorto coating.
 32. The method of claim 30, wherein prior to coating, theplurality of strands are formed into a fabric by a process selected fromthe group consisting of weaving, braiding, needle-punching, knitting andstitching.
 33. The method of claim 30, wherein the fusing comprisesapplying a pressure to the polymer particles.
 34. The method of claim33, wherein the fusing comprises applying an elevated temperature to thepolymer particles.
 35. The method of claim 30, wherein the fabric isprovided in a continuous fashion.
 36. The method of claim 35, whereinthe providing step comprises: providing a first roll for supplying acontinuous first layer of strands, wherein each strand of the firstlayer is aligned along a first direction; and providing at least asecond roll of strands for supplying a continuous second layer ofstrands positionable adjacent the first layer to form a fabric, whereineach strand of the second layer is aligned along a second direction,which is different from the first direction.
 37. The method of claim 36,further comprising a press to shape the fabric.
 38. The method of claim30, wherein an outer layer of the fabric is fused, the outer layercontaining an inner core of the particles.
 39. The method of claim 38,wherein the fabric is a heavy weight fabric.
 40. An apparatus forforming a composite fabric, comprising: a first roll supplying acontinuous first layer of strands, wherein each strand of the firstlayer is aligned along a first direction, each strand having a pluralityof fibers; at least a second roll supplying a continuous second layer ofstrands positionable adjacent the first layer to form a fabric, whereineach strand of the second layer is aligned along a second direction,which is different from the first direction, each strand having aplurality of fibers; and a reservoir containing an emulsion includingpolymer particles capable of coating substantially each fiber of thestrands of the fabric.
 41. The apparatus of claim 40, further comprisinga conveyor to carry the fabric to and from the emulsion reservoir. 42.The apparatus of claim 40, further comprising a press positioned afterthe reservoir to shape the fabric.
 43. The apparatus of claim 42,further comprising a heat source to fuse the particles coating thesubstantially each fiber to form a polymer matrix embedding theindividual fibers.
 44. The apparatus of claim 40, wherein the polymerparticles comprise a thermoplastic.
 45. A method for forming a compositearticle, comprising: providing an article having pores; exposing thearticle to a polymer emulsion including polymer particles to allow theparticles to impregnate the pores of the article and form a compositearticle which is substantially free of voids.
 46. The method of claim45, further comprising fusing the particles to form a polymer matrix inthe pores of the article.
 47. The method of claim 45, wherein thearticle is selected from the group consisting of a ceramic, wood and afoam.
 48. A composite article comprising: a porous article; and polymerparticles impregnating pores of the article to form a composite articlewhich is substantially free of voids.
 49. The article of claim 48,wherein the article is selected from the group consisting of a ceramic,wood and a foam.
 50. A composite article comprising: a porous article;and a polymer matrix embedded within pores of the article to form acomposite article which is substantially free of voids.
 51. The articleof claim 50, wherein the article is selected from the group consistingof a ceramic, wood and a foam.
 52. The article of claim 1, wherein thepolymer has an MFT of less than T_(g).
 53. The article of claim 52,wherein the polymer has an MFT of between 10° C. and 20° C. less thanT_(g).
 54. The article of claim 53, wherein the polymer has an MFT of atleast 50° C.
 55. The article of claim 54, wherein the polymer has an MFTof at least 80 ° C.
 56. The article of claim 1, wherein the particleshave an average diameter of less than 1 μm.
 57. The article of claim 1,wherein the particles have an average diameter of less than 0.5 μm. 58.The article of claim 1, wherein the particles have an average diameterof less than 0.25 μm.
 59. The method of claim 11, further comprisingdrying the particles.
 60. The method of claim 59, further comprisingfusing the particles after the step of drying.
 61. The method of claim59, further comprising fusing the outside of the article to form a fusedlayer on the article.
 62. The method of claim 20, wherein the pressureis between 207 to 276 kPa.
 63. An article, comprising: a plurality ofstrands, each strand having a plurality of fibers and substantially eachfiber being coated by particles of a polymer wherein the area betweenfibers is substantially filled by the polymer particles; and a fusedouter layer on the article.